Displays may use different image presentation techniques to produce a color image. Two general types of image presentation techniques include color matrix displays and field sequential color displays.
A color matrix display generates a color image by using a mosaic of individual color primaries. The color matrix display technique relies upon the human visual system (HVS) to spatially low pass filter the resulting mosaic image thereby mixing the primaries to achieve a full color display. In liquid crystal displays (LCDs), the color matrix is typically implemented using a color filter array. The color filter array (CFA) typically includes a patterned array of different primary color filters is placed over a display. Each of the filters only passes a limited respective spectrum of light to synthesize color primary elements. An image is generated by decomposing the image into the primaries of the CFA. The image components are then sent to the corresponding CFA components. The full color image is seen by the HVS following the visual system blending of the CFA primary images. Various CFA and backlight configurations have been used but suffer from two fundamental drawbacks. A first fundamental drawback is that energy is wasted by the light removed by the CFA elements to generate primary colors. A typical RGB primary decomposition may lose as much as ⅔ of the energy from the backlight in this filtering operation, as illustrated in FIG. 1. This reduced efficiency will result in either reduced display brightness at a given backlight power or an increase in backlight power required to achieve a specified brightness. Attempts to use an additional white primary sacrifices the display color gamut for improved display brightness and/or power efficiency. A second fundamental drawback of the CFA technique is the expense of the CFA, and additional manufacturing processes to lay down and accurately align the CFA on the display surface.
A field sequential color (FSC) display synthesizes color using a temporal mix of primaries rather than a spatial mixing of primaries, as with the CFA technique previously described. Temporal primaries are selected, such as red, green, and blue, and the image to be displayed is decomposed into the temporal primaries. The decomposition of a full color image, such as that shown in FIG. 2, into multiple temporal primaries is illustrated in FIG. 3. The full color image is displayed by temporally presenting the different individual primary images rapidly in succession. One example of FSC displays are displays that incorporate Digital Light Processing technology by Texas Instruments.
One of the principal drawbacks of the traditional FSC displays is color breakup caused by relative motion between the viewer's eye and the display. In other words, the individual primary colors (e.g., red, green, blue) are perceived separately at the edges of moving objects. The mis-registration of the color planes is due to horizontal eye motion and the display of the primary fields at temporally spaced apart times. The eye motion and different display times combine to introduce a shift of the primary images on the viewer's retina, and also result in color fringing around text. As a result, the temporal average used by the display to generate a color is disrupted causing annoying artifacts generally known as color break up.
One technique to reduce color break up is to increase the frame rate, such as from 60 Hz to 120 Hz. The increased refresh rate can reduce color break up at the expense of increased computational complexity. The increased refresh rate is also problematic for an LCD due to the relatively slow response time of the liquid crystal material. Increased color cross talk tends to result from the relatively slow liquid crystal response time thereby reducing the color gamut. Another technique to reduce color break up is to include an additional desaturated primary, such as white. The additional desaturated primary may reduce color breakup when the image content can be expressed primarily using the additional desaturated primary. In general, when image energy can be concentrated to a single primary, only one of the terms in the temporal sum is nonzero and hence there is no artifact caused by relative motion of the additional color planes. The problem arises in selecting an additional primary to match the image content. In traditional cases such as the digital light valve by Texas Instruments, the additional primary is selected at manufacture time based on expected typical content. When image content agrees with this selection color break up is reduced. When image content differs from this assumption, color break up is not effectively reduced.
Single viewer color breakup reduction techniques interactively measure the actual eye motion. The measured eye motion is used to compute an image which compensates for the difference in temporal presentation of colors. The requirement to measure the eye motion effectively limits this to applications having a single viewer in a carefully controlled position, such as a heads up display in an aircraft.
Field sequential based frame rate conversion has been used to generate fields which follow the motion of an object in the video content. In addition to the significant complexity and inevitable inaccuracy of motion estimation, the underlying assumption that the viewers' are tracking the motion of every pixel in the video is impossible to hold for a complex image scene i.e. explosion or small object motion which is not tracked and/or multiple viewers.
A temporal average of primaries to represent image color may be based upon selecting the primaries based upon image content. More specifically, one FSC technique represents a color image as a temporal sum of primary components. The LCD structure includes using a spatial grid of active RGB backlights and a color filter free LCD. The temporal primary is the product of the colored backlight and the color less LCD layer. Color break up artifacts are reduced by adapting the backlight, hence temporal primaries, locally to the image content. Additional primaries are used to refine the image color. Unfortunately, a significant limitation is the resulting computational complexity of incorporating an active spatial backlight array.